1. Field of the Invention
The present invention concerns an optical device integrated head and it particularly relates to an optical device integrated head having a structure for guiding a laser light to a recording medium in an information recording apparatus at high recording density, an optical recording apparatus having an optical recording medium and means for writing to the recording medium by a laser light, and an optical-magnetic hybrid recording apparatus having a magnetic recording medium, writing means to the recording medium by magnetic fields and means for heating the recording medium by a laser light.
2. Description of the Related Arts
Along with development of the information society in recent years, digitalization and improvement in the quality of voice and images have been progressed, and the amount of data communication in internets has been increased remarkably. In association with them, the amount of electronic data stored in servers, etc. has been increased and larger capacity for the information recording system has been demanded. Higher recording density for storing enormous amount of information without enlarging the scale of apparatus has been demanded for optical disc and magnetic disc apparatus mounted in a computer or the like as one of information recording apparatus. Higher density means refinement of a recording bit size.
In the optical disc, a method of restricting an optical spot of a laser light to a bit size by a lens is adopted. For the refinement of the spot size, shortening of the wavelength of the laser light is effective. The minimum spot diameter of a light condensed by a lens is represented by a ratio of a wavelength and the numerical aperture of a lens used for light condensation and shorter wavelength is more advantageous for higher density. However, decrease in the spot by shortening the wavelength of the laser light has a limit for increasing the density, and a finer optical spot is necessary for the bit size required for the recording density at a Tb/in2 order. For solving the problem, it has been studied for the refinement of the spot size not by condensation through the lens but by narrowing for the distance between the recording medium and the head and utilizing an optical near field. For attaining a high recording density in a magnetic disc apparatus, it is necessary to narrow the distance between the recording medium and the head and refine the crystal grain size of a magnetic film of the magnetic recording medium. Refinement of the crystal grain size in the magnetic recording medium involves a problem of making the particles thermally instable. For refining the crystal grain size and ensuring the thermal stability at the same time, it is effective to increase the coercivity. Increase in the coercivity requires increase in the intensity of a head magnetic field necessary for recording.
However, since there is a limit in the physical property of the magnetic pole material used for a recording head and narrowing of the distance between the magnetic disc and the head, it is difficult to increase the coercivity along with increase in the recording density.
For solving the problems described above, a hybrid recording technique of combining an optical recording technique and a magnetic recording technique has been proposed (refer to Hideki Saga, et al., “New Recording Method Combining Thermo-Magnetic Writing and Flux Detection”, Jpn. J. Appl, Phys., Vol. 38(1999), p. 1839-1840). During recording, a medium is heated along with generation of an application magnetic field to decrease the coercivity of the medium. This facilitates recording also to a recording medium of high coercivity for which recording was difficult by an existent magnetic head due to insufficiency of a recording magnetic intensity. For regeneration, a magnetoresistive effect used in the existent magnetic recording is utilized. Such a hybrid recording method is referred to as thermally-assisted magnetic recording or optically-assisted magnetic recording. For the optically heating mechanism, a method of restricting a laser light by a lens used in existent optical recording can be used. However, increase in the recording density of the magnetic disc apparatus has a limit in the spot diameter that can be restricted by the existent method. As a method of solution, a method of utilizing an optical near field has been proposed like in optical discs.
In the optical recording or thermally-assisted magnetic recording by using the optical near field, a laser light generated from a laser light source is guided to a recording head and used being converted into an optical spot diameter of a size and a shape suitable to recording by a device having a function of generating an optical near field (hereinafter referred to as an optical near-field transducer). Usually, among the laser light sources, a semiconductor laser diode (hereinafter referred to as laser diode) which is small in the size and decreased in the power consumption is used as the laser light source, in view of the requirement for the use in a package of a disc drive.
Both in the optical recording and thermally-assisted magnetic recording, a sufficient light intensity suitable to recording is necessary. This is an optical intensity necessary for changing the property of the material constituting bits in the optical recording and an optical intensity necessary for heating in order to lower the coercivity sufficiently to facilitate magnetization reversal of the recording medium in the thermally-assisted magnetic recording.
The optical output generated from a semiconductor laser is usually about 30 to 100 mW in a 780 nm wavelength band and a 650 nm wavelength band which are most popularized wavelength bands for optical recording light sources. Optical loss is caused till the optical output reaches the surface of the recording medium and it lowers to about several mW. An optical output about at a comparable level is necessary on the recording medium surface also in application use in the optical recording apparatus and thermally-assisted magnetic recording apparatus using the optical near field for attaining a recording density at Tb/in2 or more.
An optical near-field transducer is a device of generating a light of an extremely small spot size from a light of relatively large spot size by using a surface plasmon resonance phenomenon. In the recording density at a Tb/in2 order, the size for 1 bit is several tens nm and the size of the optical near-field transducer is about several hundreds nm.
An optical part is used for guiding a light generated from a laser diode to an optical near-field transducer. An optical loss is caused till the light generated from the laser diode is guided to the optical near-field transducer and, further, the spot size of a light restricted by a waveguide or a lens is larger compared with the size of a near-field transducer and, among the light incident to the optical near-field transducer, about from several % to ten and several % of the incident light is converted into the optical near field. Accordingly, a sufficient optical out is necessary for the laser diode when considering the optical loss caused in the course till the light reaches the recording medium. However, the intensity of light that can be generated by the semiconductor laser diode cannot be increased infinitely and it should be driven within the optical output generated at determined driving current or power consumption as the rating of the laser diode.
Optical parts for guiding the laser light generated from the laser diode to the optical near-field transducer include, for example, a reflection mirror, a lens, and an optical waveguide. The light generated from the laser diode is propagated through optical parts arranged in an optical channel and reaches an optical near-field transducer or a recording medium located anteriorly. In the course of passage through the optical channel the intensity of light decays to one/several to one/several tens of the optical output generated from the laser diode. The main cause for the decay of the light intensity includes, for example, absorption loss or scattering loss upon propagation through optical parts, coupling loss attributable to misalignment caused upon connection of optical parts (positional displacement of optical axes or difference of spot size). Such optical losses are collectively referred to as propagation loss.
For obtaining a sufficient optical intensity required for recording, it is necessary to increase the intensity of light generated from the laser diode, or decrease the propagation loss. For increasing the optical output of the laser diode, driving of the laser diode by a high current is necessary which requires increase in the output of the laser diode. However, since the intensity of light generated from the laser diode is limited, it is not practical to merely increase the optical intensity of the laser diode. This is because increase in the output of the laser diode is generally accompanied by the increase in the size of the device. Increase in the size results in remarkable increase of the power consumption and heat generation of the laser diode. Accordingly, it is an important technology to guide the light generated from a semiconductor laser efficiently to the top end of the head, that is, to decrease the propagation loss.